McNulty begins abruptly--""Find a soft place. Take a shovel and start to dig a hole""--and soon has you working your way through 4000 miles of rocks, crust, magma, mantle, and core. . . to the center of the earth where ""every direction is up"" and ""you will weigh nothing."" When you hit solid rock you exchange your shovel for a drill, and as you get deeper and hotter you switch first to an asbestos space suit and then to a super-cooled, fireproof submarine with a drill on the end of its nose. Working on through, you will come at last to the Indian Ocean--""It will be delightfully cool, but full of sharks""--and from there you can sail or paddle home. With the colors turning from red to orange to yellow as you penetrate the mantle, with a diagram of your ship and a stand-back view of your progress, this is as heady a trip--and as involving a geology lesson--as ever a curious kid stumbled into.